1. Field of the Invention
This invention relates to improved processes of introducing additives into a base product, such as sand, particularly foundry sand, and to new articles of manufacture, i.e., free flowing particles that are composed of two or more additives, the additives being bound together with a binder.
2. Background of the Invention
The addition of additives to sand, particularly to foundry sand, is a well established art. Additives such as soda lime cullet are presently being used, under the tradename of Veinguard.TM., as a foundry sand additive for the control of expansion defects in ferrous castings. Other additives known in the field include iron oxide. Under present practice, additives to foundry sands are each added independently of any other additives. By "added independently" we mean that there is a separate measuring or metering step for each additive. Alternatively, the additives are blended "dry" and then introduced into the foundry sand mix.
Whether the additives are separately introduced or dry blended with each other before introduction into the foundry sand, the additives are typically mixed dry with a core or mold sand, and then a binder is coated onto the resultant mixture. The coated mixture is placed into a pattern that gives it its final shape, and then it is cured. The cured shape, e.g., core or mold, is then used in the making of metal castings. If the sand mixing part of the process uses a continuous mixer or an automatic batch mixer, then it is likely that the dry additives will be added in an automated fashion. This will typically be accomplished by using a metering feeder. Two additives will then require two feeders, three additives will require three feeders, and so on.
Simply pre-blending the dry additives before putting them through the metering feeder may result in a loss of control over the amount of each additive that ends up in the sand mixture.
If the specific gravities of the additives are reasonably different, then separation of the pre-blended additives will readily occur. One means of reducing the likelihood of this type of separation is to reduce the particle size of the additive components to be pre-blended and then couple them using any of a variety of surface active coupling agents (silanes, for example). The small particle size of the additive reduces the mass that these surface active agents are required to hold together. The relatively weak adsorptive forces by which these surface active agents function would be overwhelmed by particles much larger than fine powders.
However, using additives that have a small particle size relative to the sand is a disadvantage. These small particles, generally referred to as fines, will reduce the core or mold strength, relative to a fixed binder level for the sand mixture and lesser amount of fines, because of an increase in the surface area that the binder is required to coat.
Another disadvantage of using an additive in the form of fine particle size, or fines, is that, as the percentage of fines increases, the more the additive is prone to generating dust when handled. Yet another disadvantage is that increasing the percentage of fines in a core or mold will decrease the ability of the core or mold to vent decomposition gases. It would be an advantage to make the particle size of the additive as close to that of the size of the sand particle as is practicable, but this will generally lead to the problem of the components separating from each other unless the components are sufficiently bound together.
Another disadvantage of adding additives to a foundry sand mix is that the pH of the sand mix is a factor affecting curing of some foundry sand binders. Additives having a pH near one end or other of the pH scale will affect the rate of cure of some binders. Accordingly, it would be advantageous if the additive added to foundry sand had a neutral pH.